1. The Field of the Invention
This invention relates to a melting process for a particulate material in which the main heat source is microwave energy.
2. The Relevant Technology
UK Patent No: 2122859A, UKAEA, discloses the use of microwave energy to heat a material, such as a glass, in a container having a cooled outer surface, the arrangement being such that a layer of melted and re-solidified material, known as a xe2x80x9cskullxe2x80x9d, is formed in contact with the internal surface of the container. Whilst the skull protects the container wall and avoids reactions between it and the melt, the container cannot easily be cleaned as the material adheres to the walls. Furthermore, start-up may be slow due to poor microwave heating of the materials to be melted at low temperature.
UK Patent No: 2228476 VERT Ltd. discloses a cold-top melter furnace in which a blanket of unmelted glass frit is maintained above the molten glass, the blanket thickness assisting in retaining volatiles. However, infrasound energy is specifically used to prevent the formation of a skull of solidified glass. This ensures that the molten glass is in contact with the furnace wall, and reactions may occur as a result.
It is an object of the present invention to avoid the disadvantages of the two known methods.
According to the invention apparatus for melting a fusible material comprises:
a microwave cavity;
means for cooling the exterior of the cavity;
means for supplying the fusible material to be melted to the interior of the cavity;
a crucible within the cavity and spaced from its walls by a layer of unmelted material, said crucible being formed from melted and re-solidified material; and
means for supplying microwave energy to the cavity of such power that fusible material in the interior of the crucible is melted.
The material to be melted may be in particulate form and/or liquid form. The particles may be between 0.5 and 10 mm and preferably 1 to 5 mm in size and/or have a volume of 1 mm3 to 100 mm3.
The material to be melted may be a preformed material, such as glass particles. The material to be melted may comprise materials to be melted to form a further material. For instance glass forming materials may be added to the cavity. Glass forming materials may include sand, sodium carbonate, lime or calcium carbonate. The sand may be replaced wholly or partially by other acidic oxides, such as B2O3 or P2O5 and/or with potassium, lithium, alkali earth metal or lead oxides.
The material to be melted may be fed to the cavity together with other materials which are not intended to be melted by the cavity. Thus higher temperature materials may be introduced and dispersed in a melt of the melted materials. The materials to be melted may comprise a matrix forming material and a material to be dispersed within that matrix. A glass matrix in which waste material is dispersed may be provided in this way. The material may be calcined or partially calcined prior to feeding.
The cavity may be microwave tunable. The cavity may be at least partially microwave tuned by its physical dimensions. The cavity may have four side walls, a bottom and top wall. The cavity may be provided in substantially spherical configuration. The cavity may have an internal volume of 2 to 500 liters and is preferably 10 to 300 liters.
The material from which the crucible has been formed may be the same as the fusible material to be melted within the crucible, and/or the same as the unmelted material. Different fusible materials may be used Preferably the crucible is formed of the same material as the unmelted material, the crucible being formed from melted and re-solidified such material. Preferably, the unmelted material remains substantially as fed to the cavity. The crucible is preferably liquid impermeable. The layer forming the crucible may be between 0.5 and 10 cm thick and is preferably 1 to 5 cm thick. The thickness of the material forming the crucible may vary between different locations. Preferably the crucible is substantially ovoid in shape. Preferably the crucible is spaced from the walls defining the cavity by the unmelted material over at least 80% of its surface area. Levels of 90 and 95% are to be preferred and most preferably no contact between the crucible and the cavity walls occurs. Preferably the crucible is made of glass.
Preferably the unmelted material is in particulate form with voids between individual particles. Preferably the unmelted material is of the same material as the crucible. The unmelted material may be provided in particles of between 1 and 5 mm. Preferably the unmelted material remains free to move relative to other portions of the unmelted material and most preferably relative to the crucible. Preferably the unmelted material provides a layer between the crucible and the cavity wall. Preferably the layer is at least 1 cm thick and more preferably 3 cm thick over at least 90% of the surface area of the crucible.
The material to be melted may be fed to the cavity by feed means such as a hopper. Preferably the fusible material feed means are provided above the cavity. Gravity feed may be relied upon to convey the fusible material into the cavity. Preferably the material is fed on to the top of the skull. Preferably the material is kept topped up within the cavity, most preferably contact between the top cavity wall and the unmelted material is maintained. Sensing means may be provided in the material feed means to determine the level of feed material. The feed means may introduce the feed into the cavity by means of a passage. Microwaves may also be introduced into the cavity by means of this passage. Preferably a column of feed material is maintained in the feed means, in gaseous contact with the cavity. In this way the column feed material can act as a filter for off gases from the melt.
The means for cooling the exterior of the cavity may comprise radiation and/or convection and/or conduction of energy away from the exterior cavity surface. Additional means may be provided to supplement the natural cooling of the cavity. One or more heat exchangers may therefore be provided in proximity to the exterior of the cavity. Heat exchangers may be provided inside the cavity and/or inside the cavity wall and/or in thermal contact with the exterior of the cavity. The provision of the heat exchanger means in contact with the exterior of the cavity is preferred for simplicity of construction. The heat exchanger may employ forced air or other forced fluid flow, such as water. Preferably the cooling means comprise one or more pipes in contact with the exterior surface of the cavity. Preferably the flow of fluid through the heat exchanger is variable. In this way the cooling extent can be varied as desired.
Preferably the microwave energy source is separated from the cavity by a fluid impermeable barrier, permeable to microwaves. Alumina, quartz, polythene or other barrier materials may be employed.
The microwave source may have a power of between 10 and 50 Kw. Preferably the power output from the microwave source is controllable.
The microwave source and/or cavity may be provided with tuning means. Preferably coarse tuning means are provided for the cavity. The coarse tuning means may be provided in a passage leading from the cavity. The cavity and passage may be separated by a fluid impermeable barrier which is permeable to microwaves. A tuning stub may be employed.
Coarse tuning means for the cavity may be provided, preferably in the form of moveable shutters. Preferably the shutters are presented in the microwave guide leading to the cavity. This microwave guide may also serve as the feed route for the fusible material.
Preferably there is further provided further means for supplying energy to material/particles within said cavity and/or crucible. The further energy means may be used during the crucible formation process and/or during tapping of the molten core. Preferably the further means are located/generated below the cavity. The further means may extend into the cavity.
The further means may be a plasma, a plasma being formed when gas-filled voids within the crucible are exposed to microwave radiation. The gas may be drawn into the cavity during draining and/or specifically introduced and/or formed in the cavity. A loading cavity may be generated in the upper part of the crucible, by melting the appropriate area of the crucible, due to plasma formation in a void formed by draining the core. Such a plasma may be used to provide initial heating of particles within the cavity. A plasma torch is preferably formed in the exit aperture from the cavity. The plasma torch may be formed by feeding a gas jet, such as an argon stream, to the exit resonant cavity. The plasma torch may be used to drain melted material from the lower part of the crucible.
Alternatively, the further means may be a microwave source acting on lossy material which have been added to the cavity. A lossy material is heated through ohmic or dielectric effects of the microwaves. Such further means are preferred during initial heating of the cavity contents. The lossy material may be graphite and/or components of the waste material to be processed or vitrified. Graphite rods and/or blocks and/or powder may be used.
The further means may be an induction heater, for instance of the radio frequency type. The induction heater may be used to melt material in the lower part of the crucible, above the exit, to tap the molten core. The induction heater may be in the form of a inductor round a metal collar, the collar extending through the aperture into the bottom of the cavity. Preferably cooling means are provided in conjunction with the metal collar. Preferably the collar is spaced from the cavity wall by an insulating material, such as a ceramic. The ceramic may be spaced over the substantial part of its area from the cavity wall also, the spacing being maintained by a limited area rib present on the insulating member.
The further means may comprise means for providing preferential conduction between a first and a second location within the cavity. Such means are particularly preferred for use during tapping process. Preferential conductors may be in the form of metallic or other thermally conductive elements, such as graphite, which are positioned between the hotter molten core of the cavity and the cooler material surrounding the exit. In this way preferential heat conduction from a core towards the exit could be provided, melting the material near the exit and so tapping the core. Graphite rods are particularly preferred for this function.
According to a second aspect of the invention, we provide a method for providing apparatus for melting material comprising:
supplying a fusible material to the interior of a microwave cavity;
cooling the exterior of the cavity;
supplying microwave energy to the cavity of such power that material within the cavity melts to form a melt pool spaced from the cavity walls by unmelted material;
causing a portion of the melted material to re-solidify around the melt pool; and
whereby a crucible formed from melted and re-solidified material is provided within the cavity and spaced from its walls by a layer of unmelted material.
Preferably the fusible material is in particulate form. Preferably the fusible particles are glass particles or glass forming materials.
Preferably the cavity is microwave tunable. Preferably the cavity is tuned to provide maximum microwave absorbency at the centre of the cavity.
Preferably the re-solidification is caused by removing the microwave energy input. Alternatively or additionally the re-solidification may be caused by decreasing the microwave energy input. Alternatively or additionally the re-solidification may be caused by increasing the exterior cooling of the cavity.
The crucible may be formed from the same material as the layer of unmelted material.
The centre of the melt pool may be allowed to drain from the cavity. It is preferred, however, that the melt pool only be partially drained from the cavity. Preferably draining the cavity leads to the introduction of further fusible material to the top of the cavity. The further feed material may be heated due to plasma formation in the void left by the material drained from the cavity. Preferably the particles at the centre of the cavity are initially heated by use of a plasma torch, preferably generated when an inert gas such as argon is supplied to the cavity and/or exit. The microwave energy may be applied to the cavity simultaneously with the plasma and/or plasma torch or alternatively the microwave energy may be applied after a portion of the material in the cavity becomes molten. Initial heating may be affected by applying microwave energy to lossy materials present within the microwave cavity. The lossy materials may be introduced into the microwave cavity along with the fusible material feed.
The melt pool may be tapped by melting the material between the melt pool and an exit aperture in the cavity. The melting of this material may be affected by the application of a plasma torch. Alternatively or additionally the melting of this material may be affected by an induction heater. Alternatively or additionally the melting of this material may be affected by preferential conduction of heat away from the melt pool towards the exit aperture.
The apparatus for melting particulate material may subsequently be used for melting similar or different fusible particles or materials or liquids. For example it may be used to process a high purity glass such as an optical glass, or a high melting point glass. Alternatively, the apparatus may be used to vitrify nuclear waste provided as a mixture of glass frit and calcined nuclear waste particles, or the material to be processed can be a mixture of waste materials and glass making materials.
Other features of the method for providing the apparatus are derivable from the first and third aspects of the invention and from the features described elsewhere in this application.
According to a third aspect of the invention we provide a method of melting a fusible material comprising supplying particles of material to the interior of a microwave cavity, the materials entering a crucible formed from melted and re-solidified material and spaced from the walls of the cavity by a layer of unmelted material, in which the material fed to the cavity and crucible is melted, energy being supplied to the crucible in the form of microwave energy, the melted material subsequently being tapped from the crucible.
The fusible material may be provided in preformed form, such as glass particles or may be added as the ingredients for a material to be formed, for instance sand, sodium carbonate, lime or calcium carbonate for forming glass. The fusible material may, therefore, be formed by the method of melting. Other materials which are not intended to be melted, but dispersed within the melt may be added.
Preferably the microwave energy applied to the cavity is tuned. Preferably the microwave energy is tuned to be preferentially absorbed within the crucible, and most preferably towards the centre of the crucible.
The fusible material may be the same or different to the re-solidified material forming the crucible.
The fusible material may be fed to the cavity under gravity feed conditions. Preferably a level of feed material is maintained over the crucible such that the top portion of that feed material is below 100xc2x0 C. In this way volatiles seeping out of the melt will be condensed on the feed material before reaching the top of the feed material.
Preferably the fusible material is introduced through the top of the cavity. Preferably the microwaves are introduced through the top of the cavity. Most preferably both the feed and microwaves are introduced through the same passage way.
The exterior cooling is preferably performed by means of heat exchangers. A series of pipes wrapped around the cavity walls forms a particularly preferred method of cooling the exterior. Preferably water is passed through these pipes, most preferably forced through. The rate of cooling applied to the exterior of the cavity is preferably variable.
The method may include the provision of further energy input to the fusible material. Further energy input may be provided by means of a plasma and/or plasma torch and/or by means of an induction heater and/or by means of lossy materials introduced into the cavity. Introduction of lossy materials and/or the provision of a plasma torch below the crucible is particularly preferred during initial heating of the fusible material.
The additional energy input may assist in the tapping of molten material from the crucible. The use of a plasma torch below the crucible and/or of an induction heater below the crucible is particularly preferred in this regard. Alternatively or additionally preferential conduction of heat away from the molten core, towards the tapping aperture may be used to melt the material below the crucible and so tap the molten core in that way.
A plasma may be used to assist in melting the top portion of the crucible. The plasma may form in a void formed as the melt pool to at least partially drained. This process assists in feeding new frit to the crucible.
Other features and steps for the method of melting are set out in the first and second aspects of this invention and discussed elsewhere in the description.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.